The present invention relates to a medical appliance in general, and more particularly to a variable-volume medical appliance. Still more particularly, the invention relates to a medical appliance having a variable volume and permitting the aspiration into and the expulsion from an internal chamber of the appliance, of a fluid medium. The appliance is advantageously disposable, i.e., intended for a single use.
Variable-volume disposable medical appliances of the type with which the present invention is concerned serve to draw fluids into an internal chamber and/or to expel fluids therefrom. Conventionally, this is accomplished with syringes, of which the needle is inserted into the tissue or into a blood vessel of a human or animal organism and either suction is produced to withdraw a quantity of fluid or pressure is produced to expel from the interior chamber of the syringe a fluid which is to be injected into the organism, for example a medication or the like. The conventional syringes can be used many times. However, between each use a conventional syringe must be sterilized. This often creates problems in terms of time, facilities or even adequacy of the sterilization. The prior art has therefore proposed a piston-type syringe which is intended for a single use, i.e., it is to be used once and then disposed of. These, however, are relatively expensive and other less expensive solutions are therefore desirable in this field. Moreover, syringes of the conventional type --whether they be reusable or of the disposable type-- create problems if they are to be used by a patient himself, as is often the case, for example with diabetics who must carry out their own injections. Such persons must draw up a sufficient quantity of medication into the syringe from a supply, and being unskilled have often been found to either exceed the required dosage or not to reach it at all.
Another type of prior art syringe, which was conceived to be disposable and does not make use of a piston structure, provides a cylinder which is hollow and from whose interior a fluid is expelled by exerting pressure upon the exterior of the cylinder to compress the same in transverse direction. The disadvantage encountered in this prior-art syringe or medical appliance resides in the fact that although it is capable of ejecting a fluid and injecting it into a human or animal organism or the like, it is incapable of aspirating a fluid, for example of drawing blood from a blood vessel. Therefore, in the case of an intended intravenous injection, this type of appliance cannot be used to determine, prior to injection of the contents, whether the needle is actually properly located in a vein, which is a test conventionally done by drawing some blood out of the vein into the syringe and, when it is found that such blood is in fact present and that therefore the needle is in fact located in the vein as required, to inject the contents of the syringe. This test is, however, also necessary in almost all other instances, e.g., also in the intra-muscular and the subcutaneous types of injection.
In these latter two types of injection, a medication is injected not into a blood vessel but into a body tissue and, since many medications intended for subcutaneous or intramuscular application can lead to disastrous effects when they are erroneously injected intravenously, it is mandatory to ascertain, e.g., by aspiration, that the tip of the injection needle is not in a blood vessel. When the needle is not located in a blood vessel, no blood appears in the syringe when suction is applied.
The significance of this test for the correct position of the injection needle is illustrated by the cases of certain diabetics who, subsequent to erroneously intravenous self-injection of insulin, which should have been injected subcutaneously, came to their deaths in a state of hypoglycemia. The test for the correct position of the injection needle by aspiration is conventional clinical procedure and it can be performed with piston-type syringes; however, it requires a change of hand position between aspiration and injection; this manipulation, especially when it is performed by laymen, can alter the originally correct position of the injection needle. Moreover, laymen often neglect to perform the aspiration test, and until now no syringe has become known which allows injection only after this test has been performed.